Sargeras could likely defeat any individual entity in WoW single-handedly, with the POSSIBLE exception of other members of the Pantheon.

“Do not lose time on daily trivialities. Do not dwell on petty detail. For all of these things melt away and drift apart within the obscure traffic of time. Live well and live broadly. You are alive and living now. Now is the envy of all of the dead.” ~ Emily3, World of Tomorrow

Medivh's only power came from Sargeras. So you're almost asking who would win: Sargeras or a fraction of Sargeras's power?

Wrong in so many ways. Medivh has power of the guardians - which is not be turn away at - he is immensely powerful. That power is passed down from guardian to guardian over thousands and thousands of years. Sargeras possessing him did indeed make him even more powerful. But that's can't take away his already extreme power that was his to start with. Could he beat sargeras in one on one combat - I'd say he has the best chance of any other Warcraft char by far. His power is purely to protect our world from such demons.

Medivh could win

If you read the lore - Medivh battles with sargeras all the time and it's a 50/50 split who actually comes on on top. Medivh never actually got fully possessed by sargeras as he was fighting him internally trying to keep him down. Medivh did plenty of good in the world - the only sargaers that came thru was the odd chance in his tower when he would go slightly insane - like when he used magic on Kadgar when he saw him in the library and didn't realize who he was cause at the time sargeras was coming thru so medivh couldn't stop him.

The other only thing that Sargeras managed - granted it was a biggy - was bringing the horde to Azeroth thru medivh

Wrong in so many ways. Medivh has power of the guardians - which is not be turn away at - he is immensely powerful. That power is passed down from guardian to guardian over thousands and thousands of years. Sargeras possessing him did indeed make him even more powerful. But that's can't take away his already extreme power that was his to start with. Could he beat sargeras in one on one combat - I'd say he has the best chance of any other Warcraft char by far. His power is purely to protect our world from such demons.

Medivh could win

If you read the lore - Medivh battles with sargeras all the time and it's a 50/50 split who actually comes on on top. Medivh never actually got fully possessed by sargeras as he was fighting him internally trying to keep him down. Medivh did plenty of good in the world - the only sargaers that came thru was the odd chance in his tower when he would go slightly insane - like when he used magic on Kadgar when he saw him in the library and didn't realize who he was cause at the time sargeras was coming thru so medivh couldn't stop him.

The other only thing that Sargeras managed - granted it was a biggy - was bringing the horde to Azeroth thru medivh

That was Sargeras' "essence," not full-fledged molten titan Sargeras that roflpwns planets.

“Do not lose time on daily trivialities. Do not dwell on petty detail. For all of these things melt away and drift apart within the obscure traffic of time. Live well and live broadly. You are alive and living now. Now is the envy of all of the dead.” ~ Emily3, World of Tomorrow